Co-channel interference is often experienced in cellular networks. It is an undesirable phenomenon that tends to limit radiofrequency (RF) coverage in portions of the RF spectrum that are shared by neighboring sectors or cells. (Hereinafter, the terms “sector” and “cell” will be used interchangeably.)
In LTE and other technologies that are based on FDMA, it is prohibited for any two mobile user terminals within the same cell to simultaneously use the same frequency or set of frequencies, unless multiuser MIMO techniques are being used. (Hereinafter, we will use the term “user equipment”, i.e., “UE”, to refer to a mobile user terminal, and we will use the term “base station” interchangeably with the term “NodeB”.) As a consequence, all of the co-channel interference experienced in LTE (and in certain other network technologies) is contributed by activity from the neighboring sectors.
In particular, the uplink (UL) interference perceived by a given sector is due to the UEs that are active in the neighboring sectors. In the nomenclature of LTE, a metric referred to as Interference over Thermal (IoT) is used to quantify the interference level.
A precise definition of IoT involves the thermal noise floor np over the bandwidth of a physical resource block (PRB), which is the basic unit of time-bandwidth resources that are allocated to the air interface in LTE. The bandwidth of a PRB is generally taken to be 180 kHz. Assuming a typical thermal noise density of −174 dBm/Hz and a typical receiver noise figure of 4 dB, a typical value of np is −118 dBm.
IoT is defined with reference to a given sector s, a given PRB having index m, and a given subframe having index j. The k'th interfering UE contributes a received interference power pm,jr(k,s) related to the power pm,jt(k,s) transmitted by UE k according to:pm,jr(k,s)=pm,jt(k,s)·ρm,j(k,s),where ρm,j (k,s) is the link gain on PRM m in subframe j from the interfering UE k to the sector s.
Accordingly, a wideband IoT, averaged over the entire operating bandwidth, is defined by:
            IoT      ⁡              (                  j          ,          s                )              =                                        ∑                          m              =              1                        M                    ⁢                                    ∑              k                        ⁢                                          p                                  m                  ,                  j                                r                            ⁡                              (                                  k                  ,                  s                                )                                                    +                  (                      M            ·                          n              p                                )                            M        ·                  n          p                      ,where the inner summation is taken over all interfering UEs and the outer summation is taken over the bandwidths of the full number M of PRBs. (In a 10-MHz LTE channel, for example, M=50.)
Often, it will be advantageous to time-average the above figure to obtain an average value IoTavg using, e.g., a single pole IIR filter with an appropriate time constant as the averager.
It is common for networks to operate in the interference-limited regime, where IoT is relatively high. Under such conditions, increasing the uplink transmit power does not generally improve spectral efficiency or system performance. Instead, the SINR levels seen on the uplinks by the base stations in neighboring sectors are pushed down with increasing IoT, driving the UEs in the neighboring sectors to increase their own transmit powers. The resulting increase in interference pushes SINR levels back down in the original sector.
Several techniques have been developed in response to the challenges posed by inter-sector cochannel interference. These include the use of upper limits on the transmit power, static frequency reuse, and soft fractional frequency reuse. Another known technique is Inter-cell Interference Control (ICIC), which typically calls for communication and feedback between base stations over the X2 interface and active load sharing between sectors. Each of these techniques has advantages and disadvantages. One disadvantage of ICIC, for example, is a relatively high level of complexity.
Another known technique of interference management is Fractional Power Control (FPC). Because FPC is managed independently within each sector, it is generally less complex than ICIC. FPC operates in an outer power control loop for uplink transmissions. In LTE, it applies to the Physical Uplink Shared Channel (PUSCH).
Very briefly, an inner power control loop steps the uplink transmit power up or down in an effort to match the actual signal-to-noise-and-interference ratio (SINR) to a target SINR. The target SINR is set in the outer power control loop, which operates on a slower time scale than the inner loop. Typically, the target SINR is computed in a modem card at the NodeB.
According to the FPC technique, the target SINR for a given UE is less, the closer the UE is to the cell edge. More precisely, the target SINR varies between a maximum and a minimum value. Between those limits, the target SINR depends on the path loss that has been estimated for the given UE. As the path loss increases, the target SINR is reduced.
Such an approach is generally useful for mitigating interference effects, because on average, it accepts a relatively low SINR from the UEs nearest the cell edge. As a consequence, the inner control loop forces the transmit power of those UEs toward a relatively low value. Because the UEs nearest the cell edge are the most potent potential interferors, managing their transmit power in such a manner tends to reduce intercell interference.
A set of parameters determine how fast the target SINR will roll off as the path loss of a given UE increases. Conventionally, these parameters are static, and as a consequence are typically set in anticipation of high loading conditions in which the IoT is also relatively high. What is lacking is greater flexibility to optimize the FPC parameters.